The overall objectives of this project are to gain some understanding of the structure and development of vertebrate visual system neurons and their connections. Adult cat retinas are fixed in glutaraldehyde and examined by electron microscopy. An unusually large, previously undescribed somato- somatic synapse between amacrine and bipolar cells has been described. Two classes of horizontal cells has been found and their ultrastructure will be described. The presence of two classes of horizontal cells, one with neurofilaments and one without, correlates well with observations by B. B. Boycott on Golgi and reduced silver stained cat retinas. Other features of cat retina to be included are observations on outer plexiform synapses, inner nuclear layer fibers, and interplexiform cells. Study of developing human retinas by electron microscopy and the Golgi and reduced silver methods will continue. Observations so far have shown the time of development of inner and outer plexiform layer synapses and the unusual fact that these synaptic contacts develop before the receptor outer segments. Studies will begin on the developing visual system of Xenopus. These will include studies on protein synthesis and the effects of protein synthesis inhibitors, colchicine, and cytochalasin-B on the developing retino-tectal projection.